Passenger cabins of present aircraft are normally air conditioned by using compressed air taken from the compressor stages of the main engines or the auxiliary engine and then cooled in air cycle cooling packs. This fresh air is mixed with filtered cabin exhaust air, which is drawn by blowers from the underfloor space. Then the mixed airstream is directed into the conditioned air distributor ducts. Such a mixing and distribution system is described in French Patent Publication 2,642,724 for a one passenger deck aircraft. Furthermore it is known, that carbon dioxide removal from the recirculation air is proposed as a future potential for further compressed air reduction.
In contrast to conventional aircraft to date, future high capacity aircraft will comprise two passenger decks that extend the entire length of the aircraft and that are interconnected by stairways. If the presently known air conditioning systems as described above, using one centralized air conditioning station in the underfloor area, were to be adapted to such a two deck aircraft, the result would require transporting great air quantities over long distances, requiring space consuming duct systems. Furthermore, it must be avoided, that used air from the upper deck enters the lower deck through the stairways, causing uncomfortable drafts and a degradation of air quality in the lower deck. Furthermore, the capability of modern engines with high bypass ratio to supply high bleed air quantities at high pressures is reduced. A reduction of the compressed air demand from the engines is therefore required for such a high capacity aircraft below that of present aircraft. Moreover, a comfort problem exists in present aircraft during heating cases, where the warm air is blown into the cabin only at ceiling level, causing uncomfortable low floor temperatures.